freeFEM.org
Science, Math, Numerical Analysis, Software - FreeFEM. Free general PDE solver driven by a user friendly language.
https://freefem.org/
GNU Octave Repository
Octave-Forge is a collection of packages providing extra functionality for GNU Octave.
https://octave.sourceforge.net/
The Netlib
Science, Math, Numerical Analysis, Software - Netlib. Searchable repository of free numerical software.
https://www.netlib.org/
DOS Beowulf
Simple, inexpensive Beowulf supercomputing clusters for DOS
https://dosbeowulf.tripod.com/
PARI/GP Development Headquarters
Science, Math, Number Theory, Software - PARI-GP Download Area. Latest versions in source and compiled form.
http://pari.math.u-bordeaux.fr/